#background {
    position: absolute;
    width: 100%;
    height: 100%;
    z-index: -1;
    top:  0;
    left: 0;
    overflow: hidden;
}

#background canvas {
    width: 100%;
    height: 100%;
}
#btns {
    position: absolute;
    width: 100%;
    height: 100%;
    z-index: 0;
    top:  0;
    left: 0;
    overflow: hidden;
}

#btns canvas {
    width: 100%;
    height: 100%;
}

